1. Field of the Invention
The invention relates to the precise and rapid determination of the orientation of a mobile in space.
2. Discussion of the Background
Typically, maritime navigation requires a knowledge of the heading followed by a ship, whether it be underway, manoeuvring or at rest. The heading information is information regarding the orientation of the longitudinal axis of the ship in a horizontal plane. The invention will be described chiefly in connection with the determination of the heading of a ship or of an aircraft, but it is applicable to other information regarding orientation of a mobile, including outside the horizontal plane: roll and pitch attitude information is another important piece of information, in maritime navigation and aerial navigation alike, although it generally has less need for precision than the heading information. The information regarding heading or orientation of a land vehicle is also important. Other exemplary applications may be envisaged such as information regarding orientation of, a crane, a weapon, etc.
The measurement of heading is traditionally performed with the aid of a magnetic compass, a gyroscopic compass or an inertial unit.
The magnetic compass requires corrections, due to the difference between geographical north and magnetic north, and due to the perturbing magnetic masses around the compass.
The gyroscopic compass, sustained electrically, is insensitive to magnetic variations, and is very widely used in ships. However, the difficulties of installation, the reduction in precision at high latitudes, mechanical servicing and the expense when high precision is desired, are considerable handicaps. Inertial units deliver a quality measurement but are expensive and have to be periodically reset with the aid of other means of measurement.
The use may also be envisaged, for the determination of orientation, of means for measuring position by satellites, using for example the radio signals transmitted by the satellites of the GPS (Global Positioning System) system.
By using several reception antennas located at different points of the ship (or of the aircraft), and by determining through differential measurements the relative positions of the antennas, it is possible to determine the attitude in terms of heading, roll, and pitch. The precision obtained under favourable conditions of reception may be 3 milliradians in heading and 6 to 8 milliradians in roll or pitch, as a function of the distance between the antennas.
However, hitherto a considerable time (several minutes) was necessary in order to obtain the desired precise information. This is no impediment for a ship making way out at sea, but it may be crippling under other conditions, for example for a ship manoeuvring in the area of a port where the reception of signals from satellites is frequently cut off by obstacles (bridges, cranes, etc.).